Optical apparatus and aberration correcting element for correcting aberration by independent control of phase distribution and defocus pattern variables

ABSTRACT

Even in the case where there is used technique such as “high NA”, “multi-layer recording”, etc., in order to carry out optimum correction of wave front aberration produced thereby with a simple technique, such an aberration correcting element to vary variable A and variable B(A≠B) of phase distribution formula A(−r 4 )−B(−r 2 ) as phase correction pattern by aberration correcting element is realized by using a liquid crystal element ( 30 ). The liquid crystal element ( 30 ) forms such an electrode pattern to generate phase distribution corresponding to spherical aberration at one transparent electrode ( 31 A), and forms such an electrode pattern to generate defocus pattern at the other transparent electrode ( 31 B). In addition, by controlling applied voltages with respect to these electrodes, it becomes possible to independently carry out variable control of the above-described variables A and B.

TECHNICAL FIELD

This invention relates to an optical head for carrying out, e.g., atleast one of recording and reproduction of light signal, an opticalapparatus provided with such an optical head, and an aberrationcorrecting element used in such optical head.

BACKGROUND ART

In recent years, realization of high recording density and/or largerecording capacity of optical recording media represented by opticaldisc is being advanced. For example, there has been put to practical use“DVD (Digital Versatile Disc: Trade Name)” (hereinafter referred to asDVD)” in which disc having the same diameter as that of “CD (CompactDisc: Trade Name (hereinafter referred to as CD)” where, e.g., objectlens (objective) numerical aperture (NA) of the optical pick-up unit is0.45, wavelength of a beam (laser beam) for signal read-out is 780 nm,disc transmission base thickness (which refers to thickness of lighttransmission layer provided on recording layer of optical disc) is 1.2mm and recording capacity is about 650 MB is used, object lens(objective) numerical aperture (NA) of optical pick-up unit is caused tobe 0.60, wavelength of a beam (laser beam) for signal read-out is causedto be 650 nm, disc transmission base thickness is caused to be 0.6 mm,and recording capacity is enhanced to 4.7 GB which is about seven timesgreater than that of CD.

Further, in this DVD, in order to approximately double recordingcapacity, two-layer recording in which two layers are provided atspacing of several ten μm is also realized.

As a technology which serves as key in realization of high recordingdensity and large recording capacity as stated above, there are “highNA” of object lens (objective) and “multi-layer recording” in theoptical recording medium.

However, in realization of “high NA” or “multi-layer recording”, theretake place problems as described below.

First, according as numerical aperture (NA) of the object lens(objective) becomes greater, spherical aberration produced by deviationquantity Δt from reference value of disc transmission base thicknessincreases in proportion to fourth power of numerical aperture (NA).Namely, quantity produced of spherical aberration is indicated as below.[Spherical aberration]∝Δt{(n²−1)/n³}NA⁴/λ

In the above formula, n is refractive index of disc base, and λ iswavelength of a beam (laser beam) for signal read-out. Namely, asunderstood from the above formula, according as “high NA” is realized,error quantity tolerated with respect to the disc transmission basethickness is remarkably decreased.

Here, when ratios of deviation quantities Δt from the reference valuetolerated with respect to the disc transmission base thickness whenquantity produced of spherical aberration is assumed to be constant inconnection with three cases of the above-described CD and DVD, andcomparative example in which further high density is assumed where“numerical aperture (NA)=0.85 and a beam wavelength=405 nm” arecalculated, there are obtained results as described below.

When Δt at CD (NA=0.45, λ=780 nm) is assumed to be 1,

Δt at DVD (NA=0.60, λ=650 nm) is equal to 0.264, and

Δt at the comparative example (NA=0.85, λ=405 nm) is equal to 0.0409.

Tolerable deviation quantity Δt from the reference value is 0.264 timesat DVD with respect to CD, and is 0.155 times at the comparative examplewith respect to DVD. Namely, it is understood that tolerable deviationquantity Δt from the reference value is decreased to so far as about1/25 as compared to CD in the condition of the comparative example.

In addition, in the “multi-layer recording” as effective system ofrealization of high recording capacity, plural layers different in thedisc transmission base thickness are intentionally provided in a stackedmanner. For this reason, quantities produced of spherical aberration atthe convergent point become different values every respective layers.

When attempt is made to carry out “high NA” or “multi-layer recording”,etc. in order to realize high recording density and large recordingcapacity as stated above, characteristic degradation based on increasein spherical aberration produced resulting from error of the disctransmission base thickness becomes problem also in both cases.

On the contrary, e.g., as disclosed in the Japanese Patent ApplicationLaid Open No. 269611/1998 publication, there is proposed a technique forforming spherical aberration correction pattern by using liquid crystalpanel, as shown in FIG. 1, in order that there result optimumaberrations every respective layers in carrying out “multi-layerrecording” to carry out aberration correction.

In this FIG. 1, the abscissa indicates radial position in the case wherenormalization is made so that radius in the correcting element for abeam converged onto the recording layer of the optical recording mediumbecomes equal to 1, and the ordinate indicates phase change quantitygiven to a beam by the aberration correcting element.

Moreover, FIG. 2 shows phase change quantity given to a beam for signalread-out by the aberration correcting element in a manner classifiedinto phase change quantity for correction of spherical aberration andphase change quantity for defocus correction, wherein the abscissaindicates radial position in the case where normalization is made sothat radius at the correcting element for the beam converged onto therecording layer of the optical recording medium becomes equal to 1, andthe ordinate indicates phase change quantity given to the beam by theaberration correcting element.

There exists the relationship that when difference between two phasechange quantities shown in this FIG. 2 is taken, pattern of phase changequantity as shown in FIG. 1 is obtained. When attempt is made to giveonly phase change quantity for correction of spherical aberration to thebeam by the correcting element without including phase change quantityfor defocus correction, its phase difference becomes large. For thisreason, phase change quantity is given in the state including phasechange quantity for defocus correction.

Namely, the phase distribution shown in FIG. 1 corresponds to thedistribution obtained by taking difference between phase distribution(−r⁴) corresponding to spherical aberration and defocus pattern (−r²)separately shown in FIG. 2, and is frequently used in carrying outaberration analysis, etc. in an ordinary sense.

FIG. 3A is an explanatory view showing the procedure in the case whereboth focus bias value and spherical aberration correction quantity thatthe aberration correcting element gives are optimized by using the phasedistribution shown in FIG. 1. Here, FIG. 3A represents signalcharacteristic by contour line, wherein the ordinate indicates phasecorrection quantity that the aberration correcting element gives, andthe abscissa indicates focus bias value. In addition, FIG. 3B is anexplanatory view showing change of spherical aberration quantity anddefocus quantity adjusted by the phase correction quantity and the focusbias value shown in FIG. 3A by using the coordinate axis intelligiblymodified, wherein the ordinate indicates spherical aberration quantityand the abscissa indicates defocus quantity.

Here, in the case where liquid crystal panel is used as an aberrationcorrecting element 4 and there is employed a configuration to correctspherical aberration by aberration correction pattern as shown in FIG. 1as disclosed in the Japanese Patent Application Laid Open No.269611/1998 publication, problems as described below take place.

It is to be noted that phase distribution close to the pattern shown inFIG. 1 is caused to be generated in a pseudo manner by step-shapedpattern based on division of electrode pattern in the Japanese PatentApplication Laid Open No. 269611/1998 publication. On the other hand,even if such step-shaped pattern is not employed, technology forgenerating continuous phase distribution is announced in, e.g., “4p−K−1of proceedings of academic lecture meeting of autumn society of appliedphysics, 2000” or “CPM 2000-91 (2000-09) “Technical Research Report ofInstitute of Electronics and Communication Engineers of Japan” Societyof Electronic Information and Communication”, etc. In this technology,electrodes positioned at the inner circumferential side and the outercircumferential side of the liquid crystal panel are used to generateelectric field in a direction along the principal surface in place ofthickness direction of the panel to form potential gradient in thedirection of the panel surface within the liquid crystal layer. However,even if such a liquid crystal panel which generates continuous phasedistribution is used as the aberration correcting element, problem asdescribed below similarly takes place.

Namely, in the optical recording medium using object lens (objective) of“high NA” or “multi-layer recording”, etc., in the case where the signalcharacteristic is optimized, it is necessary to optimize both focus biasvalue and spherical aberration correction quantity by the liquidcrystal.

However, in the case where attempt is made to carry out suchoptimization by using the aberration correction pattern of FIG. 1, itcan be confirmed that the signal characteristic when focus bias valueand correction quantity by liquid crystal are changed results in contourline distribution as shown in FIG. 3A.

Accordingly, in the case where adjustment is carried out from the“initial position” toward the “best position” in the figure, if “settingof focus bias” and “setting of liquid crystal correction quantity” arenot alternatively repeated many times, it is impossible to follow up sothat there results the “best position”.

This not only allows the adjustment to be complicated, but also leads tothe fact that adjustment is converged into the point which is not the“best position” by a little factor.

This can be considered in a manner as described below.

For the purpose of simplifying the explanation, the signalcharacteristic with respect to defocus quantity and spherical aberrationis assumed to be a characteristic as shown in FIG. 3B. Here, as thesignal characteristic, amplitude of RF signal or jitter of RF signal,etc. may be used.

As described above, by first changing “focus bias”, it is possible toadjust “defocus quantity” without affecting “spherical aberration”.

However, “spherical aberration” can be corrected by “phase control byliquid crystal”, and, on the other hand, gives change also to formationof light spot on a light detecting element for forming focus errorsignal and intensity distribution by “phase change by liquid crystal”produced when spherical aberration quantity is controlled. For thisreason, followed by “phase control by liquid crystal”, “focus bias”where signal characteristic becomes satisfactory would change (Inpractice, the best image surface position where the signalcharacteristic becomes best also somewhat changes by “sphericalaberration control”.

Accordingly, in the case where the “focus bias” is caused to beconstant, and the “spherical aberration quantity” is caused to bechanged, defocus quantity also changes followed by change of “sphericalaberration quantity”. However, phase distribution corresponding toessential aberration required in correcting spherical aberrationproduced by error of the disc transparent base thickness is only thephase distribution (−r⁴) corresponding to the spherical aberration.

Further, change of aberration quantity in the case where the phasedistribution shown in FIG. 1 is used to carry out aberration correctioncan be expressed in a manner described below by using variable C.[Pattern 0]=C{(−r ⁴)−(−r ²)}  (1)

Conventional adjustment can be expressed as the case where it is assumedthat this variable C is changed.

Namely, in the conventional correction, the phase distribution shown inFIG. 1 is collectively changed in the ordinate direction by variable C.

As stated above, in the case where correction is made by usingaberration correction pattern disclosed in this Japanese PatentApplication Laid Open No. 269611/1998, there was the problem thatadjustment of correction quantity becomes complicated.

DISCLOSURE OF THE INVENTION

In view of the above, an object of this invention is to provide anaberration correcting element in which even in the case where techniquesuch as “high NA” or “multi-layer recording”, etc. is used for thepurpose of realization of high recording density or large recordingcapacity, it can correct, in an optimum manner, wave front aberration(mainly spherical aberration) produced thereby with a simple technique,an optical head using such an aberration correcting element, and anoptical apparatus using such an optical head.

To attain the above-mentioned object, an optical head according to thisinvention is directed to an optical head for carrying out at least oneof recording and reproduction of an information signal with respect toan optical recording medium including a light transmission layer on arecording layer where the information signal is recorded, the opticalhead comprising: a light source for emitting a beam; converging meansfor converging the beam onto the recording layer of the opticalrecording medium; light detecting means for detecting a reflected beamconverged onto the recording layer of the optical recording medium bythe converging means and reflected by the recording layer; andaberration correcting means provided on an optical path extending fromthe light source to the converging means for controlling, by anarbitrary pattern, spherical aberration and defocus of the beamconverged onto the recording layer of the optical recording medium.

Moreover, an optical apparatus according to this invention is directedto an optical apparatus for carrying out at least one of recording andreproduction of an information signal with respect to an opticalrecording medium including a light transmission layer on a recordinglayer where the information signal is recorded, the optical apparatuscomprising: an optical head for irradiating a beam with respect to theoptical recording medium and for detecting a reflected beam from therecording layer of this optical recording medium, a servo circuit forcontrolling the optical head on the basis of a light detection signaloutputted from this optical head, and a signal processing circuit forprocessing the light detection signal outputted from the optical head,wherein the optical head includes a light source for emitting a beam,converging means for converging the beam onto the recording layer of theoptical recording medium, light detecting means for detecting thereflected beam converged onto the recording layer of the opticalrecording medium by the converging means and reflected by this recordinglayer, and aberration correcting means disposed on an optical pathextending from the light source to the converging means and forcontrolling, by an arbitrary pattern, spherical aberration and defocusof the beam converged onto the recording layer of the optical recordingmedium.

Further, an aberration correcting element according to this invention isdirected to an aberration correcting element which can be disposed on anoptical path within an optical head for carrying out at least one ofrecording and reproduction of an information signal with respect to anoptical recording medium including a light transmission layer on arecording layer where the information signal is recorded, wherein whenradius of beam spot of the beam converged onto the recording layer is rand variables different from each other are A, B, the transmitted beamis caused to generate phase distribution indicated by the followingphase distribution formula:A(−r⁴)−B(−r²).

In the optical head of this invention, aberration correcting means forcontrolling, by an arbitrary pattern, spherical aberration and defocusof the beam with respect to the recording layer of the optical recordingmedium is provided on the optical path extending from the light sourceto the converging means. Thus, even in the case where there is employedtechnique such as “high NA” or “multi-layer recording”, etc. forrealization of high recording density and/or large recording capacity,it becomes possible to correct, in an optimum manner, wave frontaberration (mainly spherical aberration) produced thereby with a simpletechnique.

Moreover, in the optical apparatus of this invention, aberrationcorrecting means for controlling, by an arbitrary pattern, sphericalaberration and defocus of the beam with respect to the recording layerof the optical recording medium is provided on the optical pathextending from the light source to the converging means. Thus, even inthe case where there is employed technique such as “high NA” or“multi-layer recording”, etc. for realization of high recording densityand/or large recording capacity, etc., it becomes possible to correct,in an optimum manner, wave front aberration (mainly sphericalaberration) produced thereby with a simple technique.

Further, in the aberration correcting element of this invention, whenradius of beam spot of the beam converged onto the recording layer, andvariables different from each other are A, B, the transmitted beam iscaused to generate phase distribution indicated by the following phasedistribution formula:A(−r⁴)−B(−r²).Thus, even in the case where there is employed technique such as “highNA” or “multi-layer recording”, etc. for realization of high recordingdensity and/or large recording capacity in the optical head or theoptical apparatus, it becomes possible to correct, in an optimum manner,wave front aberration (mainly spherical aberration) produced therebywith a simple technique.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory view showing an example of spherical aberrationcorrection pattern in a conventional aberration correcting element.

FIG. 2 is an explanatory view showing phase distribution given to alaser beam by the aberration correcting element shown in FIG. 1 in thestate classified into phase distribution for spherical aberrationcorrection and phase distribution for defocus correction.

FIGS. 3A and 3B are explanatory views showing procedure in the casewhere phase correction pattern of the aberration correcting elementshown in FIG. 1 is used to optimize both focus bias value and sphericalaberration correction quantity of aberration correcting element byliquid crystal element by using the phase correction pattern of theaberration correcting element shown in FIG. 1.

FIG. 4 is a block diagram showing the configuration of an opticalapparatus using optical disc in which aberration correcting element andoptical head are assembled in the best mode for carrying out thisinvention.

FIG. 5 is an explanatory view showing outline of optical system of theoptical head shown in FIG. 4.

FIG. 6 is an explanatory view showing a little practical example of theconfiguration of the optical head shown in FIG. 4.

FIGS. 7A, 7B and 7C are explanatory views showing the configuration ofliquid crystal element provided at the optical head shown in FIG. 4.

FIG. 8 is an explanatory view showing a more practical example of phasecorrection pattern which can be generated by using the liquid crystalelement shown in FIG. 7.

FIG. 9 is an explanatory view showing the procedure in the case wherethe phase correction pattern shown in FIG. 8 is used to optimize focusbias value and spherical aberration correction quantity.

FIG. 10 is an explanatory view showing a technique for determining valueof correction ratio K=B/A between phase distribution corresponding tospherical aberration and defocus pattern in the optical head shown inFIG. 4.

FIG. 11 is a graph showing the relationship between the shortest marklength and correction ratio K in the optical disc.

FIG. 12 is a plan view showing the state of a diffracted beam spot bythe shortest mark on optical disc.

FIGS. 13A, 13B and 13C are graphs showing differences of aberrationcorrection quantities by values of correction ratios K (In FIG. 13A,K=1; In FIG. 13B, K=1.25; In FIG. 13C, K=1.5).

FIG. 14 is a graph showing the relationship of spherical aberrationcorrection quantity, focus bias and signal characteristic.

FIG. 15 is a front view showing shape of light receiving surface oflight detector in optical head.

FIGS. 16A and 16B are side views showing the states of an incident beam(FIG. 16A) and a reflected beam (FIG. 16B) with respect to optical discwhen spherical aberration correction is made toward the (−) side in thesystem where degree of contribution of high NA light with respect tofocus error signal is high.

FIGS. 17A and 17B are side views showing the states of an incident beam(FIG. 17A) and a reflected beam (FIG. 17B) with respect to optical discwhen spherical aberration correction is made toward the (+) side in thesystem where degree of contribution of high NA light with respect tofocus error signal is high.

FIG. 18 is a side view showing the state of an incident beam and areflected beam with respect to optical disc when spherical aberrationand focus bias are in optimum state.

FIGS. 19A and 19B are side views showing an incident beam (FIG. 19A) anda reflected beam (FIG. 19B) with respect to optical disc when sphericalaberration correction is carried out toward the (−) side in the systemwhere degrees of contribution of high NA light and low NA light withrespect to focus error signal are the same.

FIGS. 20A and 20B are side views showing the states of an incident beam(FIG. 20A) and a reflected beam (FIG. 20B) with respect to optical discwhen spherical aberration correction is carried out toward the (+) sidein the system where degrees of contribution of high NA light and low NAlight with respect to focus error signal are the same.

FIGS. 21A, 21B and 21C are side views showing the state of an incidentbeam with respect to optical disc when spherical aberration correctionis carried out in the case where switching with respect to opticalrecording medium different in thickness of light transmission layer iscarried out.

BEST MODE FOR CARRYING OUT THE INVENTION

Best modes for carrying out an optical head, an optical apparatus and anaberration correcting element according to this invention will now bedescribed in detail on the basis of attached drawings.

It is to be noted that best modes which will be explained below arepreferred embodiments of this invention, and technically preferablevarious restrictions are attached, but the scape of this invention isnot limited to these embodiments as long as there does not exist thedescription to the effect that this invention is particularly limited inthe following description. Namely, while rotationally operated opticaldisc is used as the optical recording medium in the following bestmodes, various media may be used without being limited to the opticaldisc as this optical recording medium. In addition, while the opticalapparatus of this invention is constituted, in the following best modes,as an apparatus which carries out recording and reproduction of aninformation signal with respect to the optical recording medium, theoptical apparatus of this invention may be constituted as a recordingapparatus which carries out only recording of the information signalwith respect to the optical recording medium, or a reproducing apparatuswhich carries out only reproduction of the information signal from theoptical recording medium.

FIG. 4 is a block diagram showing the configuration of an opticalapparatus using optical disc in which an aberration correcting elementand an optical head are assembled in the best mode for carrying out thisinvention. It is to be noted that the optical apparatus using opticaldisc shown in FIG. 4 is an example of an optical apparatus in whichaberration correcting element and optical head which will be describedbelow can be assembled.

This optical apparatus 101 comprises, as shown in FIG. 4, a spindlemotor 103 for rotationally driving an optical disc 102, an optical head104, a feed motor 105 for this optical head 104, and circuit blockswhich will be described later.

Here, as the optical disc 102, there may be employed an optical discwhich copes with “high NA (Numerical Aperture)” of object lens(objective) or “multi-layer recording”. The spindle motor 103 is causedto undergo drive control by a system controller 107 and a servo controlunit 109 so that it is rotated to rotationally operate the optical disc102.

The optical head 104 respectively carries out light irradiation withrespect to the signal recording surface of the rotating optical disc 102in accordance with commands of the system controller 107 and a signalmodulation/demodulation and ECC block 108. By such light irradiation,recording and/or reproduction with respect to the optical disc 102 arecarried out. Moreover, this optical head 104 is supported so thatmovement operation by the feed motor 105 for moving this optical head104 up to a desired recording track on the optical disc 102 can becarried out.

Further, the optical head 104 detects various beams as described lateron the basis of a reflected beam from the signal recording surface ofthe optical disc 102 to deliver signals corresponding to a respectivebeam to a pre-amplifier unit 120.

The pre-amplifier unit 120 generates focus error signal, tracking errorsignal and RF signal, etc. on the basis of signals corresponding to therespective beam to send the RF signal to the signalmodulation/demodulation and ECC block 108, and to send respective errorsignals to the servo control unit 109.

The signal modulation/demodulation and ECC block 108 carries outmodulation/demodulation and addition of ECC (Error Correcting Code) ofthe sent RF signal in accordance with kind of optical disc 102 to bereproduced in dependency upon control of the system controller 107.

The system controller 107 detects kind of loaded optical disc 102 and/orwhether or not corresponding recording is multi-layer recording througha recording surface detecting unit 106. Moreover, the system controller107 controls drive of liquid crystal panel serving as an aberrationcorrecting element provided at the optical head 104 through a liquidcrystal driver 110.

The recording surface detecting unit 106 detects surface reflectionfactor and/or other difference in shape or outer appearance of opticaldisc 102, etc. to thereby detect recording system/kind of loaded opticaldisc 102, and/or relative linear velocity between the recording surfaceand the beam, etc., and to detect either one of divided recording areasin the case where the recording area of this optical disc 102 isdivided, or either one of stacked recording surfaces in the case wherethis optical disc 102 has plural stacked recording surfaces.

Moreover, the servo control unit 109 controls the spindle motor 103, theoptical head 104 and the feed motor 105 in accordance with sentrespective error signals. Namely, control of the spindle motor 103,control of the feed motor 105 and controls in focussing direction and intracking direction of the biaxial actuator which holds object lens(objective) of the optical head 104 are respectively carried out by theservo control unit 109.

For example, if the optical disc 102 is disc for data storage ofcomputer, recording signal demodulated at the signalmodulation/demodulation and ECC block 108 is sent out to an externalcomputer 130, etc. through an interface 111. Thus, the external computer130, etc. can receive, as reproduction signal, signal recorded at theoptical disc 102.

Moreover, if the optical disc 102 is audio/visual disc, reproductionsignal is caused to undergo digital conversion at D/A converting unit ofa D/A, A/D converter 112, and is delivered to an audio/visual processingunit 113. Further, the reproduction signal in which audio/visual signalprocessing has been implemented at this audio/visual processing unit 113is transmitted to external audio/visual equipment such as motion pictureprojector, etc. through an audio/visual signal input/output unit 114.

It is to be noted that it is sufficient that either one of the D/A, A/Dconverter 112 and the audio/visual processing unit 113 is provided inaccordance with use purpose of the optical apparatus, and it is notnecessarily required that both units are provided.

Further, in the case of carrying out recording of the information signalwith respect to the optical disc 102 by this optical apparatus, signaldelivered from the external computer 130, or external audio/visualequipment is sent to the signal modulation/demodulation and ECC block108 via the interface 111 or the D/A, A/D converter 112. This signalmodulation/demodulation and ECC block 108 modulates sent signal inaccordance with control of the system controller 107 to control lightsource of the optical head 104 through a laser control circuit 115 onthe basis of the modulated signal. In addition, emission output of thelight source of the optical head 104 is modulated. Thus, recording ofthe information signal with respect to the optical disc 102 is carriedout.

The optical head 104 comprises, as shown in FIG. 5, a light source 2, abeam splitter (polarization beam splitter) 3, an aberration correctingelement 4, a quarter wavelength plate 5, an object lens (objective) 6,and a light detecting element 7, and is constituted in such a mannerthat these respective optical parts are individually mounted. As thelight source 2, semiconductor laser is used.

In this optical head 104, the beam emitted from the light source 2 isincident on the beam splitter 3, and is transmitted through thisreflection surface because such beam is P-polarized light with respectto the reflection surface of this beam splitter 3. Thus, aberration isrendered by the aberration correcting element 4. Further, such beam istransmitted through the quarter wavelength plate 5, and are focused on acertain point on the signal recording surface of the optical disc 102and are irradiated therefrom.

Numerical aperture NA of the object lens (objective ) 6 is caused to be,e.g., 0.65 or more. In addition, the optical disc 102 may be a disc suchthat at least two recording layers or more are provided as previouslydescribed.

The reflected beam from the signal recording surface of the optical disc102 is transmitted through the object lens (objective) 6, the quarterwavelength plate 5 and the aberration correcting element 4 for a secondtime, and are incident on the beam splitter 3. Such reflected beam isreflected at this reflection surface because it is S-polarized lightwith respect to the reflection surface of this beam splitter 3. Thereflected beam is branched from the optical path returning to the lightsource 2, i.e., is separated from beam emitted from the light source 2,and is received by the light detecting element 7.

Further, by using an output signal from this light detecting element 7,reproduction of signals recorded on the optical disc 102 and generationof respective error signals are carried out. In addition, by the beamirradiated onto the optical disc 102, recording of the informationsignal with respect to this optical disc 102 is carried out.

In more practical sense, as shown in FIG. 6, this optical head 104 iscaused to be of the configuration comprising a semiconductor laserelement 12 serving as a light source, collimator lens 13, 19, apolarization beam splitter 14, a liquid crystal element 15 serving as anaberration correcting element, a quarter wavelength plate 16, an objectlens (objective) 17, a light detecting element 18 for FAPC (Front AutoPower Control), a beam splitter 20, and light detecting elements 21, 22.

Namely, in this optical head 104, the beam emitted from thesemiconductor laser element 12 is changed into substantially parallelbeam by the collimator lens 13, and is incident on the polarization beamsplitter 14.

The polarization beam splitter 14 branches the beam emitted from thesemiconductor laser element 12 into the beam for monitoring laser lightintensity and the beam for carrying out recording or reproduction ofsignals.

The beam from the semiconductor laser element 12 separated by thepolarization beam splitter 14 and transmitted therethrough is caused toundergo rendering of aberration as described above by the liquid crystalelement 15 serving as the aberration correcting element of the best modefor carrying out this invention, and are passed through the quarterwavelength plate 16. Thus, such beam is focused on a certain point ofthe signal recording surface of the optical disc 102 by the object lens(objective) 17, and is irradiated therefrom.

The reflected beam from the signal recording surface of the optical disc102 is incident for a second time on the polarization beam splitter 14through the object lens (objective) 17, the quarter wavelength plate 16and the liquid crystal element 15. The beam which is incident on thepolarization beam splitter 14 for a second time is reflected at thereflection surface of this polarization splitter 14, and is incident onthe collimator lens 19 so that the focused beam is provided. Suchfocused beam is branched by the beam splitter 20 so that it is receivedby a pair of light detecting elements 21, 22 for obtaining a focus errorsignal by, e.g., the so-called “spot size method”, and are received bythese light detecting elements 21, 22.

By using output signals from these light detecting elements 21, 22,generation of servo signal including focus error signal and reproductionof signals recorded on the optical disc 102 are carried out.

Further, the aberration correcting element 4 in this best mode gives, tothe transmitted beam, phase difference of the pattern represented by thefollowing formula (2) by using variable A and variable B which aredifferent from each other by the notation similar to the previouslydescribed formula (1).[Pattern 1]=A(−r ⁴)−B(−r ²)  (2)

(In the above formula, A≠B)

Further, this aberration correcting element 4 is characterized in thatit can change both variable A and variable B or one of them in thisformula (2). Such aberration correcting element for changing variable Aor variable B can be realized by, e.g., liquid crystal element of theconfiguration as shown in FIG. 7A.

As shown in FIG. 7A, liquid crystal element 30 is caused to be of theconfiguration in which liquid crystal molecules 34 are sealed betweentwo glass bases (substrates) 31A, 31B. At the insides (surfaces oppositeto each other) of the respective glass bases 31A, 31B, transparentelectrodes 32A, 32B for applying voltage onto the liquid crystalmolecules 34 are provided. In addition, at the insides (surfacesopposite to each other) of the respective transparent electrodes 32A,32B, orientation films 33A, 33B for giving orientation to the liquidcrystal molecules 34 are provided. Here, arrow a in FIG. 7A indicatesorientation direction (rubbing direction) of the orientation films 33A,33B.

In such liquid crystal element 30, in the state where voltage is notapplied to the respective transparent electrodes 32A, 32B, liquidcrystal molecules 34 are disposed in parallel with the respectiveorientation films 33A, 33B along the orientation direction given by therespective orientation films 33A, 33B. In addition, when voltage isapplied to the respective transparent electrodes 32A, 32B, liquidcrystal molecules 34 rise in a direction perpendicular to the respectiveorientation films 33A, 33B. In this instance, it is possible to controlrising angle of liquid crystal molecules 34 by level of applied voltage.

Further, in this best mode, in the aberration correcting element 4comprised of the liquid crystal element 30, when voltage is applied tothe liquid crystal molecules 34 by the electrodes 32A, 32B, refractiveindex of this liquid crystal molecule layer changes on the basis ofapplied voltage.

It is to be noted that respective transparent electrodes 32A, 32B may beformed as divided electrodes as described in the Japanese PatentApplication Laid Open No. 269611/1998 to apply different voltages to thedivided respective electrodes to control the voltage distribution tothereby control phase distribution given to the transmitted beam. Inaddition, as described in the previously mentioned “4p−K−1 ofproceedings of academic lecture meeting of autumn society of appliedphysics, 2000” or “CPM 2000-91 (2000-09)” Technical Research Report ofInstitute of Electronics and Communication Engineers of Japan” Societyof Electronic Information and Communication”, etc., electrodespositioned at the inner circumferential side and the outercircumferential side of the liquid crystal panel may be formed toproduce electric field in a direction along the principal surface inplace of thickness direction of the panel to form potential gradient inthe panel surface direction within the liquid crystal layer to generatecontinuous phase distribution.

Further, in this best mode, at one transparent electrode 32A, as shownin FIG. 7B, such an electrode pattern to generate phase distributioncorresponding to spherical aberration is formed, and at the othertransparent electrode 32B, as shown in FIG. 7C, such an electrodepattern to generate defocus pattern is formed. In addition, bycontrolling applied voltages with respect to these electrodes, it ispossible to independently carry out variable control of theabove-described variables A and B.

It is to be noted that the aberration correcting element 4 comprised ofthis liquid crystal element 30 may be disposed in such a manner that thebeam from the light source is incident from the side of one transparentelectrode 32A, or may be disposed in such a manner that the beam fromthe light source is incident from the side of the other transparentelectrode 32B.

Further, two liquid crystal elements or more may be used in a stackedmanner to thereby realize generation of phase distribution as describedabove.

Further, the example where liquid crystal element is used as theaberration correcting element 4 has been described in this best mode,but the aberration correcting element 4 is not limited to suchimplementation. The aberration correcting element 4 may be constitutedalso by using phase change material, e.g., PLZT (ferroelectric,piezoelectric or electro-optical ceramic material consisting of zirconicacid lead titanate lanthanum), etc.

Further, the phase correction pattern can be diversely changed asmentioned in FIG. 8, for example, by changing ratio between variables Aand B. In this case, in FIG. 8, in order to indicate change of patternshape, the pattern shapes are indicated by changing K (correction ratio)of[Pattern 1]=A{(−r ⁴)−K(−r ²)}in the condition where A=1 and K=B/A.

Further, although the above-described “change of optimum focus bias byspherical aberration correction” changes also in dependency upon themethod of forming focus error, if suitable K is selected, distributionof the signal characteristic with respect to the “focus bias value” and“phase correction quantity A” is permitted to be the distribution inwhich adjustment is easy as shown in FIG. 9. Thus, adjustment procedurefrom the initial position to the best position can be simplified and canbe accurate.

The technique for determining value of correction ratio K=B/A betweenphase distribution corresponding to spherical aberration and defocuspattern will now be briefly described by using FIG. 10. In the statewhere optical disc is loaded into the optical apparatus to turn ON lightsource of the optical head, the following operation will be carried out.

(1) First, “focus bias value” is swung (changed) from the initialposition to determine best point (or tolerance center point) of thesignal characteristic.

(2) Then, aberration correction by the phase distribution correspondingto the spherical aberration is carried out to add spherical aberrationby A(−r⁴) (It is to be noted that direction of addition is caused to bethe direction where signal characteristic is improved).

(3) Then, defocus adjustment quantity by defocus pattern is swung in thestate where the spherical aberration quantity and the focus bias valueare caused to be constant so that the signal characteristic becomes bestin that state (adjustment quantity is B(−r²)).

(4) By the above-mentioned steps, K=B/A is determined. A is changed inthe state where this K is kept constant, i.e., voltages of bothelectrodes of phase distribution corresponding to spherical aberrationand defocus pattern are controlled so that the signal characteristicbecomes best (or tolerance center).

It is to be noted that while, here, with respect to the case whereoptimum value of K is unknown, derivation technique thereof has beendescribed, value of this K can be approximately determined if theconfigurations of the optical disc and the optical head are determined.In that case, aberration correction pattern corresponding to value ofdesired K may be provided as one transparent electrode of the aberrationcorrecting element (liquid crystal element), and the other transparentelectrode may be caused to be fixed electrode (entire electrode). Thus,the number of pins for driving the liquid crystal element can bereduced.

In addition, when aberration correction pattern corresponding to valueof desired K is provided at one transparent electrode, and defocuspattern is provided in advance at the other transparent electrode tochange defocus value in the state where K is kept constant, only appliedvoltage of one transparent electrode may be caused to have distributionto use function of defocus pattern of the other transparent electrode asoccasion demands.

Optimization of the spherical aberration correction pattern will befurther considered below.

When roughly classified, the following two kinds of optimum patterns areconceivable.

(1) Pattern in which “optimization (adjustment) of correction quantity”is easy with respect to a certain optical disc.

(2) Pattern in which FOCUS BIAS is not changed at the time of switchingbetween multi-layer recording layers in the optical disc.

Here, first, consideration will be made in connection with what valueoptimum correction ratio K between spherical aberration (r⁴ term) anddefocus (r² term) takes in the case where the positional relationshipbetween object lens (objective) and recording surface of the opticaldisc is not changed.[Correction pattern]=A{(−r ⁴)−K(−r ²)}

It is to be noted that judgment of the state where the signalcharacteristic becomes best with respect to changes of sphericalaberration and/or defocus, etc. is carried out on the basis of jitter ofRF signal, error rate and/or RF signal amplitude, etc. The applicant ofthis application has experimentally obtained the experiment result thatthe method in which the state where amplitude of the shortest markbecomes maximum is taken as reference has good sensitivity and hasrelatively small deviation with respect to the center of margin amongthese methods. Here, the term called the shortest mark designates 3 Tmark in, e.g., EFM modulation, and designates 2 T mark in (1, 7)modulation.

When [λ/(NA×[shortest mark length]×2)] is taken as the abscissa andvalue of optimum K is taken as the ordinate as value which representsthe state of diffraction pattern serving as basis of modulation by theshortest mark, it is seen that a predetermined relationship exists therebetween as shown in FIG. 11.

Here, the optimum K is K where change of the shortest mark amplitudebecomes minimum in the case where size is changed and A is changed inthe state where shape of pattern is maintained with respect toaberration corresponding to shape of correction pattern in order tocorrect spherical aberration. Accordingly, when the shortest mark lengthchanges, value of the optimum K would change.

The trend of the graph shown in FIG. 11 can be interpreted as follows.Namely, according as the shortest mark becomes smaller, value of[λ/NA×[shortest mark length]×2)] becomes greater, and overlap of the0-th order light and the ±1-th order light diffracted by succession ofthe shortest marks results in the area corresponding to the peripheralportion of aperture, i.e., the portion where numerical aperture (NA) ofthe object lens (objective) is large as shown in FIG. 12.

On the other hand, when K becomes great, the portion close to theportion where phase distribution is flat shifts to the areacorresponding to the portion where numerical aperture (NA) is large asshown in FIGS. 13A, 13B and 13C. FIG. 13A shows the phase distributionin the beam radius direction where K=1, FIG. 13B shows the phasedistribution in the beam radius direction where K=1.25, and FIG. 13Cshows the phase distribution in the beam radius direction where K=1.5.

Accordingly, it can be interpreted that it is the condition of optimum Kthat there results a phase distribution such that the position whererays of light of the area which contributes to the shortest mark areconverged is not changed so much by the aberration correction.

In practice, because focus error is also changed by change of aberrationin return light, the above-mentioned value of optimum K is not incorrespondence with the above-described “K which simplifies adjustmentof aberration correction”.

In this case, in the optical head for “DVD (Digital Versatile Disc)”,value of optimum K is about 1.35. Further, in the optical head foroptical disc of which density is caused to be higher than DVD, value ofoptimum K becomes greater according as the above-described shortest markbecomes smaller. Namely, it can be said that, in the optical head foroptical disc equivalent to DVD, or caused to have higher density, valueof optimum K is value more than 1.

Here, in the system of the shortest mark length in which the optimum Kbecomes equal to 1.15 in the above-described reference, the result inthe case where aberration correction is carried out by using liquidcrystal element of K=1 is indicated. When spherical aberrationcorrection quantity at this time is taken as the ordinate and focus biasis taken as the abscissa, the signal characteristic can be representedby contour line as shown in FIG. 14. In the case where there is carriedout spherical aberration correction of (+) of quantity in which defocuseffect by (r²) term is changed by 1 μm, the optimum focus bias wasshifted to the side away from the optical disc (Disc Far) by quantitycorresponding to 0.15 μm. In this case, employment of an approach toallow K to be equal to 0.85 means that focus bias deviation iscancelled. This point can be interpreted as follows.

Namely, in this experiment, detection of focus error signal was carriedout by the system of detecting focus error signal by the so-called“astigmatism method” in which a light detector having divided lightreceiving surfaces at the central portion is used as shown in FIG. 15 touse only light received at four light receiving surfaces of theperipheral side in the state where light received at the central portionis not used for focus error detection (the system shown in the JapanesePatent Application No. 277544/1999 that the applicant of thisapplication has already proposed (Japanese Patent Application Laid OpenNo. 101681/2001) (U.S. patent application Ser. No. 09/671103, U.S.application Date: Sept. 27, 2000, Title “Optical Head, Light DetectingElement, Light Information Recording/Reproducing Apparatus and FocalPoint Error Detection Method”. For this reason, the beam of the portionwhere numerical aperture (NA) is small (hereinafter referred to as “lowNA light” hardly contribute to change of focus error signal, and returnin-focus position of the beam of the portion where numerical aperture(NA) is great (hereinafter referred to as “high NA light”) is dominantwith respect to change of focus error signal.

Here, the case where focus error signal becomes equal to zero in thestate where quality of RF signal is best as shown in FIG. 18 will beconsidered. FIGS. 16A and 16B respectively show the state of a beambefore reflection by the optical disc and the state of a beam afterreflection by the optical disc in the case where aberration correctionof (−) is carried out. In this case, in these FIGS. 16A, 16B and 18, andFIGS. 17A, 17B, 19A, 19B, 20A and 20B which will be described later,wavy lines indicate return light outgoing (emission) focal positionwhere focus error signal is zero and quality of RF signal is best.

Namely, in the case where aberration correction of (−) is carried out,it looks that high NA light exists at the Near side with respect toin-focus position irrespective of the fact that the optical disc becomesmore distant in practice (K>1) in the state where signal is best(Outgoing (Emission) focal point position of return light is located atthe side close to the object lens (objective) with respect to theoptimum state). Thus, in the focus error signal, there results thatsignal best point is shifted to the Near side. FIG. 16A shows the stateof the beam before reflection, and FIG. 16B shows the state of the beamafter reflection.

Further, FIGS. 17A and 17B respectively show the state of a beam beforereflection by the optical disc and the state of a beam after reflectionby the optical disc in the case where aberration correction of (+) iscarried out. In the case where aberration correction of (+) is carriedout, it looks that high NA light is located at the Far side with respectto in-focus position irrespective of the fact that the optical disc isnear in practice (K>1) in the state where signal is best as shown inFIGS. 17A and 17B (Outgoing (Emission) focal point position of returnlight is located at the side far from the object lens (objective) withrespect to the optimum state. Thus, in the focus error signal, thereresults the state where the signal best point is shifted to the Farside. FIG. 17A shows the state of the beam before reflection, and FIG.17B shows the state of the beam after reflection.

This result indicates that different result is obtained in the system ofdetecting a focus error signal by a method in which the degree that thebeam within aperture contribute to focus error signal is different,which is so called “astigmatism method” or “spot size method”, etc.

Further, in practice, since there takes place change also in dependencyupon what NA of return optical path which detects focus error signal is,or how divisional width of light detector is set if “spot size method”is employed, etc., it is desirable to suitably optimize value of K inaccordance with respective designs.

As another example, in connection with the system where degrees ofcontribution with respect to focus error signal between high NA lightand low NA light are the same, the incident beam and the reflected beamare shown in a model form in a manner similar to the above in FIGS. 19A,19B, FIG. 20A and FIG. 20B. By taking into the consideration the factthat contributions with respect to focus error signal of high NA lightand low NA light are the same, such state where high NA light and low NAlight are symmetrical at the Near side or the Far side with respect tothe outgoing (emission) focal point position where the focus errorsignal becomes equal to zero after reflection is assumed. When it isassumed that an aberration which becomes so after reflection is givenbefore reflection, it is sufficient that movement quantity (shiftquantity) of in-focus position of low NA light is caused to be greater,i.e., K>1.

Additionally, FIG. 19A shows a beam before reflection by the opticaldisc 102 in the case where aberration correction of (−) is carried out,FIG. 19B shows a beam after reflection by the optical disc 102 in thecase where aberration correction of (−) is carried out, FIG. 20A shows abeam before reflection by the optical disc 102 in the case whereaberration correction of (+) is carried out, and FIG. 20B shows a beamafter reflection by the optical disc 102 in the case where aberrationcorrection of (+) is carried out.

Then, consideration will be made in connection with the case whereswitching is carried out between states where thickness of cover layeris greatly different like the case of optical disc of multi-layerrecording, etc. In this case, since spherical aberration is corrected inoptimum manner on the signal recording surface unlike theabove-described case, K such that in-focus position of the beam whichcontributes to focus error signal is not changed as far as possiblebefore and after correction is optimum K.

When consideration is made in connection with the case where a lightdetector having divided light receiving surfaces at the central portionshown in FIG. 15 is used, the beam which mainly contributes to focuserror signal is the beam of the region where NA is high in this case.Accordingly, as shown in FIGS. 21A, 21B and 21C, such a pattern tocorrect deviation of in-focus position (spherical aberration) of thebeam of the region where NA is low in the state where in-focus positionof the beam of the region where NA is high is maintained is desirable.Namely, assuming that aberration is corrected in the state where thecover layer is thin as shown in FIG. 21A, when there results the statewhere cover layer is thick as shown in FIG. 21B, in-focus position ofhigh NA light is far with respect to in-focus position of low NA light.At the time of correcting this, it is sufficient that in-focus positionof low NA light is caused to be in correspondence with in-focus positionof high NA light as shown in FIG. 21C.

Value of K which realizes this becomes about 1.25 as understood from theabove-described “Figures of portions of values of K which maintainamplitude of the shortest mark” (FIGS. 13A, 13B and 13C).

Further, also in this case, value of optimum K varies by the system ofdetecting focus error signal. In addition, in the case where optimumspherical aberration correction pattern (i.e., value of K) is consideredin a manner as described above, there are many cases in general wherepattern which facilitates “optimization (adjustment) of correctionquantity” with respect to a certain optical disc and pattern which doesnot change FOCUS BIAS in switching between multi-layers are not incorrespondence with each other.

Then, consideration will be made in the following correction patternswith respect to the case where respective patterns which can be employedin the optical head in the best mode for carrying out this invention areadapted.[Correction pattern]=A{(−r ⁴)−K(−r ²)}

As a pattern which can realize all K, the following four (pattern (A) topattern (D)) are conceivable.

Pattern (A)

One surface: A(−r⁴)

Other surface: B(−r²)

$\begin{matrix}{{{{Correction}\mspace{14mu}{pattern}\text{:}\mspace{11mu}{A\left( {- r^{4}} \right)}} - {B\left( {- r^{2}} \right)}} = {A\left\{ {\left( {- r^{4}} \right) - {\left( {B/A} \right)\left( {- r^{2}} \right)}} \right\}}} \\{= {A\left\{ {\left( {- r^{4}} \right) - {K\left( {- r^{2}} \right)}} \right\}}} \\{\left( {{\because{B/A}} = K} \right)}\end{matrix}$

In this case, K=B/A.

Pattern (B)

One surface: A(−r⁴)−B1(−r²)

Other surface: B2(−r²)

$\begin{matrix}{{Correction}\mspace{14mu}{{pattern}:{{A\left( {- r^{4}} \right)} - {B\; 1\left( {- r^{2}} \right)} - {B\; 2\left( {- r^{2}} \right)}}}} \\{= {{A\left( {- r^{4}} \right)} - {\left( {{B\; 1} + {B\; 2}} \right)\left( {- r^{2}} \right)}}} \\{= {{A\left( {- r^{4}} \right)} - {{B\left( {- r^{2}} \right)}\left( {{\because{{B\; 1} + {B\; 2}}} = B} \right)}}} \\{= {A\left\{ {\left( {- r^{4}} \right) - {\left( {B/A} \right)\left( {- r^{2}} \right)}} \right\}}} \\{= {A\left\{ {\left( {- r^{4}} \right) - {K\left( {- r^{2}} \right)}} \right\}\left( {{\because{B/A}} = K} \right)}}\end{matrix}$

In this case, K=B/A=(B1+B2)/A.

Pattern (C)

One surface: A1(−r⁴)−B(−r²)

Other surface: −A2(−r⁴)

$\begin{matrix}{{Correction}\mspace{14mu}{{pattern}:{{A\; 1\left( {- r^{4}} \right)} - {B\;\left( {- r^{2}} \right)} + {A\; 2\left( {- r^{4}} \right)}}}} \\{= {{\left( {{A\; 1} + {A\; 2}} \right)\left( {- r^{4}} \right)} - {B\left( {- r^{2}} \right)}}} \\{= {{A\left( {- r^{4}} \right)} - {{B\left( r^{2} \right)}\left( {{\because{{A\; 1} + {A\; 2}}} = A} \right)}}} \\{= {A\left\{ {\left( {- r^{4}} \right) - {\left( {B/A} \right)\left( {- r^{2}} \right)}} \right\}}} \\{= {A\left\{ {\left( {- r^{4}} \right) - {K\left( {- r^{2}} \right)}} \right\}\left( {{\because{B/A}} = K} \right)}}\end{matrix}$

In this case, K=B/A=B/(A1+A2)

Pattern (D)

One surface: A1(−r⁴)−B1(−r²)

Other surface: −A2(−r⁴)+B2(−r²)

$\begin{matrix}{{Correction}\mspace{14mu}{{pattern}:{{A\; 1\left( {- r^{4}} \right)} - {B\; 1\;\left( {- r^{2}} \right)} + {A\; 2\left( {- r^{4}} \right)} - {B\; 2\left( {- r^{2}} \right)}}}} \\{= {{\left( {{A\; 1} + {A\; 2}} \right)\left( {- r^{4}} \right)} - {\left( {{B\; 1} + {B\; 2}} \right)\left( {- r^{2}} \right)}}} \\{= {{A\left( {- r^{4}} \right)} - {{B\left( r^{2} \right)}\left( {{\because{{A\; 1} + {A\; 2}}} = A} \right)\left( {{\because{{B\; 1} + {B\; 2}}} = B} \right)}}} \\{= {A\left\{ {\left( {- r^{4}} \right) - {\left( {B/A} \right)\left( {- r^{2}} \right)}} \right\}}} \\{= {A\left\{ {\left( {- r^{4}} \right) - {K\left( {- r^{2}} \right)}} \right\}\left( {{\because{B/A}} = K} \right)}}\end{matrix}$

In this case, K=B/A=(B1+B2)/(A1+A2).

Further, as a pattern which can realize simple switching between twovalues of K, the following pattern (E) is conceivable.

Pattern (E), two surfaces are individually moved in the pattern (D)(when movement is carried out with respect to respective surfaces,movement is not carried out with respect to the other surface).

Further, as a pattern for carrying out adjustment into one K, thefollowing pattern (F) is conceivable.

Pattern (F)

One surface: A(−r⁴)−B(−r²)

Other surface: fixed electrode (entire electrode)

$\begin{matrix}{{{{Correction}\mspace{14mu}{pattern}\text{:}\mspace{11mu}{A\left( {- r^{4}} \right)}} - {B\left( {- r^{2}} \right)}} = {A\left\{ {\left( {- r^{4}} \right) - {\left( {B/A} \right)\left( {- r^{2}} \right)}} \right\}}} \\{= {A\left\{ {\left( {- r^{4}} \right) - {K\left( {- r^{2}} \right)}} \right\}}} \\{\left( {{\because{B/A}} = K} \right)}\end{matrix}$

In this case, K=B/A. Since A, B are variables with respect to the sameelectrode, K becomes fixed value.

Further, which the above-described patterns are respectively adaptedwith respect to the following four cases (case 1 to case 4) isindicated.

[Case 1] In the case where importance to only convenience of adjustmentis attached without coping with multi-layer disc, etc., it is desirableto use pattern (F).

[Case 2] In the case where there is a desire to arbitrarily set value ofK in a manner also including dispersion, etc., it is desirable to useeither one of the pattern (A) to the pattern (D).

[Case 3] In the case where, e.g., two K are required for the purpose ofadjustment and switching between plural recording layers, it isdesirable to use pattern (D).

[Case 4] In the case where setting to only one K which becomes trade offof either adjustment or switching between plural recording layers, orboth thereof is made by taking into consideration structure of theaberration correcting element and/or simplification of drive or control,it is desirable to use pattern (F).

In addition, this invention is not limited to the above-described bestmodes, but various applications and modifications are conceivable withinthe scope which does not depart from the gist of this invention.

As explained above, in accordance with the optical head of the best modefor carrying out this invention, aberration correcting element forcontrolling, by an arbitrary pattern, spherical aberration and defocusof the beam with respect to recording layer of optical recording mediumis provided between the object lens and the light source. Thus, even inthe case where there is used technique such as “high NA” or “multi-layerrecording”, etc. for high recording density or large recording capacity,it becomes possible to correct, in an optimum manner, with a simpletechnique, wave front aberration (mainly spherical aberration) producedthereby.

Further, in accordance with the optical apparatus of the best mode forcarrying out this invention, aberration correcting element whichcontrols, by an arbitrary pattern, spherical aberration and defocus ofthe beam with respect to the recording layer of the optical recordingmedium is provided between object lens of the optical head and the lightsource. Thus, even in the case where there is used technique such as“high NA” or “multi-layer recording”, etc. for high recording density orlarge recording capacity, etc., it becomes possible to correct, in anoptimum manner, with a simple technique, wave front aberration (mainlyspherical aberration) produced thereby.

In addition, in accordance with the aberration correcting element of thebest mode for carrying out this invention, when effective radius in adirection corresponding to the signal recording direction by the opticalhead is assumed to be r, phase distribution corresponding to phasedistribution formula A(−r⁴)−B(−r²) is generated with respect tovariables A, B which satisfy A≠B. Thus, even in the case where there isused technique such as “high NA”, “multi-layer recording” for highrecording density or large recording capacity, etc. in the optical heador the optical apparatus, it becomes possible to correct, in an optimummanner, with a simple technique, wave front aberration (mainly sphericalaberration) produced thereby.

1. An optical head for carrying out at least one of recording andreproduction of an information signal with respect to an opticalrecording medium including a light transmission layer on a recordingportion having at least two recording layers where the informationsignal is recorded, the optical head comprising: a light source foremitting a beam; converging means for converging the beam onto therecording layer of the optical recording medium; light detecting meansfor detecting a reflected beam converged onto the recording layer of theoptical recording medium by the converging means and reflected by therecording layer; and aberration correcting means provided on an opticalpath extending from the light source to the converging means forcontrolling, by an arbitrary pattern, spherical aberration and defocusof the beam converged onto the recording layer of the optical recordingmedium, wherein when the radius of beam spot of the beam converted ontothe recording layer is assumed to be r, and variables different fromeach other are assumed to be A and B so that B/A=K is provided, theaberration correcting means allows the beam to generate phasedistribution indicated by the following phase distribution formula:$\begin{matrix}{{{A\left( {- r^{4}} \right)} - {B\left( {- r^{2}} \right)}} = {A\left\{ {\left( {- r^{4}} \right) - {B/{A\left( {- r^{2}} \right)}}} \right\}}} \\{= {A\left\{ {\left( {- r^{4}} \right) - {K\left( {- r^{2}} \right)}} \right\}}}\end{matrix}$ where either a value of K is caused to be a value whichcancels change of the optimum value of focus bias produced by switchingof selection of recording layer of two recording layers or more in theoptical recording medium, or plural values are set in advance incorrespondence with two recording layers or more in the opticalrecording medium as value of K, and value of K corresponding to aselected recording layer is selected and is used.
 2. The optical head asset forth in claim 1, wherein the converging means has numericalaperture of 0.65 or more.
 3. The optical head as set forth in claim 1,wherein the aberration correcting means allows the variable A and thevariable B in the phase distribution formula to be changed independentlyeach other.
 4. The optical head as set forth in claim 1, wherein valueof K is set to value which cancels deviation of the optimum value offocus bias produced when spherical aberration quantity given to the beamis changed so that change quantity of focus bias value becomes minimum.5. The optical head as set forth in claim 4, wherein value of K is morethan
 1. 6. An optical head for carrying out at least one of recordingand reproduction of an information signal with respect to an opticalrecording medium including a light transmission layer on a recordinglayer where the information signal is recorded, the optical headcomprising: a light source for emitting a beam; converging means forconverging the beam onto the recording layer of the optical recordingmedium; light detecting means for detecting a reflected beam convergedonto the recording layer of the optical recording medium by theconverging means and reflected by the recording layer; and aberrationcorrecting means provided on an optical path extending from the lightsource to the converging means for controlling, by an arbitrary pattern,spherical aberration and defocus of the beam converged onto therecording layer of the optical recording medium, wherein the aberrationcorrecting means comprises refractive index adjustable means in whichrefractive index is changed on the basis of applied voltage, and a pairof electrodes for applying voltage to this refractive index adjustablemeans, the pair of electrodes including an electrode provided onrespective sides of the refractive index adjustable means; wherein whenthe radius of beam spot of the beam converted onto the recording layeris assumed to be r, and variables different from each other are assumedto be A and B so that B/A=K is provided, the aberration correcting meansallows the beam to generate phase distribution indicated by thefollowing phase distribution formula: $\begin{matrix}{{{A\left( {- r^{4}} \right)} - {B\left( {- r^{2}} \right)}} = {A\left\{ {\left( {- r^{4}} \right) - {B/{A\left( {- r^{2}} \right)}}} \right\}}} \\{= {A\left\{ {\left( {- r^{4}} \right) - {K\left( {- r^{2}} \right)}} \right\}}}\end{matrix}$ where a value of K is set to value which cancels deviationof the optimum value of focus bias produced when spherical aberrationquantity given to the beam is changed so that change quantity of focusbias value becomes minimum by one electrode, and either a value of K isset to value which cancels change of the optimum value of focus biasproduced by switching of selection of recording layer of two recordinglayers or more in the optical recording medium by the other electrode,or a value of K corresponding to a selected recording layer is selectedfrom plural values of K set in advance in correspondence with tworecording layers or more in the optical recording medium by the otherelectrode.
 7. The optical head as set forth in claim 6, wherein theaberration correcting means is comprised of liquid crystal element. 8.The optical head as set forth in claim 6, wherein voltage value appliedby the electrode has a concentrical distribution with respect to thetransmitted beam.
 9. The optical head as set forth in claim 6, whereinthe aberration correcting means is caused to be of the configuration inwhich electrodes which make a pair are provided at both sides of therefractive index adjustable means, and generates phase distributioncorresponding to [A(−r⁴)] term in the phase distribution formula by oneelectrode and generates phase distribution corresponding to [B(−r²)]term in the phase distribution formula by the other electrode.
 10. Theoptical head as set forth in claim 6, wherein the aberration correctingmeans is caused to be of the configuration in which electrodes whichmake a pair are provided at both sides of the refractive indexadjustable means, and wherein in the case where B=B1+B2 is assumed inthe phase distribution formula, phase distribution corresponding to[A(−r⁴)−B1(−r²)] term is generated by one electrode and phasedistribution corresponding to [B2(−r²)] term in the phase distributionformula is generated by the other electrode.
 11. The optical head as setforth in claim 6, wherein the aberration correcting means is caused tobe of the configuration in which electrodes which make a pair areprovided at both sides of the refractive index adjustable means, andwherein in the case where A=A1+A2 is assumed in the phase distributionformula, phase distribution corresponding to [A1(−r⁴)−B1(−r²)] term isgenerated by one electrode and phase distribution corresponding to[−A2(−r⁴)] term in the phase distribution formula is generated by theother electrode.
 12. The optical head as set forth in claim 6, whereinthe aberration correcting means is caused to be of the configuration inwhich electrodes which make a pair are provided at both sides of therefractive index adjustable means, and wherein in the case where A=A1+A2and B=B1+B2 are assumed in the phase distribution formula, phasedistribution corresponding to [A1(−r⁴)−B1(−r²)] term is generated by oneelectrode and phase distribution corresponding to [−A2(−r⁴)+B2(−r²)]term in the phase distribution formula is generated by the otherelectrode.
 13. An optical apparatus adapted for carrying out at leastone of recording and reproduction of an information signal with respectto an optical recording medium including a light transmission layer on arecording layer where the information signal is recorded, the opticalapparatus comprising: an optical head for irradiating a beam withrespect to the optical recording medium, and for detecting a reflectedbeam from the recording layer of this optical recording medium; a servocircuit for controlling the optical head on the basis of a lightdetection signal outputted from the optical head; and a signalprocessing circuit for processing the light detection signal outputtedfrom the optical head, wherein the optical head includes a light sourcefor emitting a beam, converging means for converging the beam onto therecording layer of the optical recording medium, light detecting meansfor detecting the reflected beam converged onto the recording layer ofthe optical recording medium by the converging means and reflected bythe recording layer, and aberration correcting means disposed on anoptical path extending from the light source to the converging means andfor controlling, by an arbitrary pattern, spherical aberration anddefocus of the beam converged onto the recording layer of the opticalrecording medium, wherein when radius of beam spot of the beam convergedonto the recording layer is assumed to be r, and variables differentfrom each other are assumed to be A and B so that B/A=K is provided, theaberration correcting means allows the beam to generate phasedistribution indicated by the following phase distribution formula:$\begin{matrix}{{{A\left( {- r^{4}} \right)} - {B\left( {- r^{2}} \right)}} = {A\left\{ {\left( {- r^{4}} \right) - {B/{A\left( {- r^{2}} \right)}}} \right\}}} \\{= {A\left\{ {\left( {- r^{4}} \right) - {K\left( {- r^{2}} \right)}} \right\}}}\end{matrix}$ where either a value of K is caused to be value whichcancels change of the optimum value of focus bias produced by switchingof selection of recording layer of two recording layers or more in theoptical recording medium, or plural values are set in advance incorrespondence with two recording layers or more in the opticalrecording medium as value of K and value of K corresponding to aselected recording layer is selected and is used.
 14. The opticalapparatus as set forth in claim 13, wherein the converging means hasnumerical aperture of 0.65 or more.
 15. The optical apparatus as setforth in claim 13, wherein the aberration correcting means includesmeans for changing the variables A and B in the phase distributionformula independently each other.
 16. The optical apparatus as set forthin claim 13, wherein value of K is set to value which cancels deviationof the optimum value of focus bias produced when spherical aberrationquantity given to the beam is changed so that change quantity of focusbias value becomes minimum.
 17. The optical apparatus as set forth inclaim 16, wherein value of K is more than
 1. 18. The optical apparatusas set forth in claim 13, wherein at least one of recording andreproduction of an information signal is carried out with respect to anoptical recording medium where at least two recording layers or more areprovided.
 19. An optical apparatus adapted for carrying out at least oneof recording and reproduction of an information signal with respect toan optical recording medium including a light transmission layer on arecording layer where the information signal is recorded, the opticalapparatus comprising: an optical head for irradiating a beam withrespect to the optical recording medium, and for detecting a reflectedbeam from the recording layer of this optical recording medium; a servocircuit for controlling the optical head on the basis of a lightdetection signal outputted from the optical head; and a signalprocessing circuit for processing the light detection signal outputtedfrom the optical head, wherein the optical head includes a light sourcefor emitting a beam, converging means for converging the beam onto therecording layer of the optical recording medium, light detecting meansfor detecting the reflected beam converged onto the recording layer ofthe optical recording medium by the converging means and reflected bythe recording layer, and aberration correcting means disposed on anoptical path extending from the light source to the converging means andfor controlling, by an arbitrary pattern, spherical aberration anddefocus of the beam converged onto the recording layer of the opticalrecording medium, wherein the aberration correcting means comprisesrefractive index adjustable means in which refractive index is changedon the basis of applied voltage, and a pair of electrodes for applyingvoltage to this refractive index adjustable means, the pair ofelectrodes including an electrode provided on respective sides of therefractive index adjustable means; wherein when radius of beam spot ofthe beam converged onto the recording layer is assumed to be r, andvariables different from each other are assumed to be A and B so thatB/A=K is provided, the aberration correcting means allows the beam togenerate phase distribution indicated by the following phasedistribution formula: $\begin{matrix}{{{A\left( {- r^{4}} \right)} - {B\left( {- r^{2}} \right)}} = {A\left\{ {\left( {- r^{4}} \right) - {B/{A\left( {- r^{2}} \right)}}} \right\}}} \\{= {A\left\{ {\left( {- r^{4}} \right) - {K\left( {- r^{2}} \right)}} \right\}}}\end{matrix}$ where a value of K is set to value which cancels deviationof the optimum value of focus bias produced when spherical aberrationquantity given to the beam is changed so that change quantity of focusbias value becomes minimum by one electrode, and either a value of K isset to value which cancels change of the optimum value of focus biasproduced by switching of selection of recording layer of two recordinglayers or more in the optical recording medium, or a value of Kcorresponding to a selected recording layer is selected from pluralvalues of K set in advance in correspondence with two recording layersor more in the optical recording medium by the other electrode.
 20. Theoptical apparatus as set forth in claim 19, wherein the aberrationcorrecting means is comprised of liquid crystal element.
 21. The opticalapparatus as set forth in claim 19, wherein voltage value applied by theelectrode has a concentrical distribution with respect to thetransmitted beam.
 22. The optical apparatus as set forth in claim 19,wherein the aberration correcting means is caused to be of theconfiguration in which electrodes which make a pair are provided at bothsides of the refractive index adjustable means, and generates phasedistribution corresponding to [A(−r⁴)] term in the phase distributionformula by one electrode and generates phase distribution correspondingto [B(−r²)] term in the phase distribution formula by the otherelectrode.
 23. The optical apparatus as set forth in claim 19, whereinthe aberration correcting means is caused to be of the configuration inwhich electrodes which make a pair are provided at both sides of therefractive index adjustable means, and wherein in the case where B=B1+B2is assumed in the phase distribution formula, phase distributioncorresponding to [A(−r⁴)−B1(−r²)] term is generated by one electrode,and phase distribution corresponding to [B2(−r²)] term in the phasedistribution formula is generated by the other electrode.
 24. Theoptical apparatus as set forth in claim 19, wherein the aberrationcorrecting means is caused to be of the configuration in whichelectrodes which make a pair are provided at both sides of therefractive index adjustable means, and wherein in the case where A=A1+A2is assumed in the phase distribution formula, phase distributioncorresponding to [A1(−r⁴)−B(−r²)] term is generated by one electrode,and phase distribution corresponding to [−A2(−r⁴)] term in the phasedistribution formula is generated by the other electrode.
 25. Theoptical apparatus as set forth in claim 19, wherein the aberrationcorrecting means is caused to be of the configuration in whichelectrodes which make a pair are provided at both sides of therefractive index adjustable means, and wherein in the case where A=A1+A2and B=B1+B2 are assumed in the phase distribution formula, phasedistribution corresponding to [A1(−r⁴)−B1(−r²)] term is generated by oneelectrode and phase distribution corresponding to [−A2(−r⁴)+B2(−r²)]term in the phase distribution formula is generated by the otherelectrode.
 26. An aberration correcting element which can be disposed onan optical path within an optical head for carrying out at least one ofrecording and reproduction of an information signal with respect to anoptical recording medium including a light transmission layer on arecording portion including at least two recording layers where theinformation signal is recorded, wherein when radius of beam spot of abeam converged onto the recording layer is assumed to be r, andvariables different from each other are assumed to be A and B so thatB/A=K is provided, a transmitted beam is caused to generate phasedistribution represented by the following phase distribution formula$\begin{matrix}{{{A\left( {- r^{4}} \right)} - {B\left( {- r^{2}} \right)}} = {A\left\{ {\left( {- r^{4}} \right) - {B/{A\left( {- r^{2}} \right)}}} \right\}}} \\{= {A\left\{ {\left( {- r^{4}} \right) - {K\left( {- r^{2}} \right)}} \right\}}}\end{matrix}$ where either a value of K is caused to be value whichcancels change of the optimum value of focus bias produced by switchingof selection of recording layer of two recording layers or more in theoptical recording medium, or plural values are set in advance incorrespondence with two recording layers or more in the opticalrecording medium as value of K and value of K corresponding to aselected recording layer is selected and is used.
 27. The aberrationcorrecting element as set forth in claim 26, wherein the aberrationcorrecting element is caused to be of the configuration comprisingrefractive index adjustable means in which refractive index is changedon the basis of applied voltage, and an electrode for applying voltageto this refractive index adjustable means.
 28. The aberration correctingelement as set forth in claim 27, wherein the aberration correctingmeans is comprised of liquid crystal element.
 29. The aberrationcorrecting element as set forth in claim 27, wherein voltage valueapplied by the electrode has a concentrical distribution with respect tothe transmitted beam.
 30. The aberration correcting element as set forthin claim 27, wherein the aberration correcting element is caused to beof the configuration in which electrodes which make a pair are providedat both sides of the refractive index adjustable means, and whereinphase distribution corresponding to [A(−r⁴)] term in the phasedistribution formula is generated by one electrode, and phasedistribution corresponding to [B(−r²)] term in the phase distributionformula is generated by the other electrode.
 31. The aberrationcorrecting element as set forth in claim 27, wherein the aberrationcorrecting element is caused to be of the configuration in whichelectrodes which make a pair are provided at both sides of therefractive index adjustable means, and wherein in the case where B=B1+B2is assumed in the phase distribution formula, phase distributioncorresponding to [A(−r⁴)−B1(−r²)] term is generated by one electrode,and phase distribution corresponding to [B2(−r²)] term in the phasedistribution formula is generated by the other electrode.
 32. Theaberration correcting element as set forth in claim 27, wherein theaberration correcting means is caused to be of the configuration inwhich electrodes which make a pair are provided at both sides of therefractive index adjustable means, and wherein in the case where A=A1+A2is assumed in the phase distribution formula, phase distributioncorresponding to [A1(−r⁴)−B(−r²)] term is generated by one electrode,and phase distribution corresponding to [−A2(−r⁴)] term in the phasedistribution formula is generated by the other electrode.
 33. Theaberration correcting element as set forth in claim 27, wherein theaberration correcting means is caused to be of the configuration inwhich electrodes which make a pair are provided at both sides of therefractive index adjustable means, and wherein in the case where A=A1+A2and B=B1+B2 are assumed in the phase distribution formula, phasedistribution corresponding to [A1(−r⁴)−B1(−r²)] term is generated by oneelectrode, and phase distribution corresponding to [−A2(−r⁴)+B2(−r²)]term in the phase distribution formula is generated by the otherelectrode.
 34. The aberration correcting element as set forth in claim26, which includes means for changing variables A and B in the phasedistribution formula independently each other.